1. Field of the Invention
The present invention relates to a receiver applied to a television set for receiving a digital broadcast.
2. Description of the Related Art
Conventionally, one program is broadcast by only one physical channel in analog broadcasting. In this case, the physical channel is defined as a frequency band of carrier waves used for broadcasting a program. By a receiver of analog broadcasting, a channel to be received is switched by switching a frequency to be received. Therefore, when a user tunes in to a frequency of a channel to be received, images and voices of a program, which is being broadcast in this channel, can be instantly outputted. In a receiver of analog broadcasting, in order to select a program easily, there is provided a channel-up/channel-down function by which a receiving channel can be switched in the order of an increase or decrease in the channel number.
The channel-up/channel-down function is executed when a channel-up key or channel-down key provided in a main body or remote control unit is operated.
Recently, digital broadcasting, in which a plurality of programs can be broadcast by one physical channel, for example, digital broadcasting by Advanced Television Standards Committee System (ATSC system) has started. In digital broadcasting, a plurality of multiplex virtual channels are formed in a physical channel, and a program is broadcast for each virtual channel. Concerning the receiver of digital broadcasting, it is proposed to provide the above channel-up/channel-down function so that a program can be easily selected.
In this connection, digital broadcasting is conducted while images and voices are being compressed. Therefore, in order to output the images and voices in the receiving channel, it necessary to expand the received digital broadcasting signal. Accordingly, it takes a few seconds (2 to 3 seconds) from the switching operation of the channel to the output of the images and voices of a program which is being broadcast in the indicated receiving channel. For the above reasons, when the channel-up key or channel-down key is continuously operated by a user, according to the conventional channel-up/channel-down function in which the channel tuning motion is conducted by the number of times of operation, it takes long time until the images and voices of the program, which the user has determined to watch and listen to, are outputted, that is, the operation property is not good.
In order to solve the above problems, there is proposed a receiver (Japanese Unexamined Patent Publication No. 0-42212) in which only the number of a selected channel is successively switched while the channel-up key or channel-down key is being continuously operated, and the tuning motion is not executed. In this receiver, the time at which operation of the channel-up key or channel-down key is completed, that is, the time at which the receiving channel has been decided is detected, and the tuning motion for the receiving channel is started at this timing.
In this connection, as described before, in digital broadcasting, a plurality of programs are broadcast in one physical channel. Therefore, there are two channels, one is a physical channel showing a physical channel and the other is a multiplex virtual channel. The receiving channel is selected from the virtual channel.
However, in digital broadcasting, the number of programs which are being broadcast in the physical channel, that is, the number of virtual channels is dynamically changing. Therefore, information (VCT (Virtual Channel Table)) showing attributes of all virtual channels, in the physical channel of which programs are broadcast, are included in the digital broadcasting signals at a predetermined time. When the receiver receives this VCT, it is possible for the receiver to recognize the number of programs (the number of virtual channels) which are being broadcast in each physical channel.
Specifically, a channel map, on which the virtual channels in which programs are being broadcast are registered, is stored for each physical channel, and the channel map is renewed according to the received VCT. Therefore, the channel map is renewed for each physical channel when the digital broadcasting signals, which are being broadcast in the physical channel, are received.
Usually, in the above channel-up/channel-down function, the receiving channel is successively switched according to the registration content of the channel map. Therefore, the following problems may be encountered. Unless the channel map is renewed, a virtual channel in which a program is being broadcast is not tuned in, or a virtual channel in which a program is not being broadcast is tuned in, that is, the tuning operation becomes complicated.
In the receiver disclosed in Japanese Unexamined Patent Publication No. 10-42212, when the receiving channel is switched by the channel-up/channel-down function, the tuning motion is not carried out until a user decides the receiving channel. Therefore, the channel map is not renewed at this time (The digital broadcasting signal is not received.). Accordingly, there is a high possibility that the above problems are caused. Since the tuning motion is started after the receiving channel has been decided, it takes long time from the completion of switching the receiving channel to the output of images of the program which is being broadcast in the receiving channel.